When considering the inspection of sheet materials such as paper, plastics or textiles it is desirable to differentiate between mechanical and optical defects. Mechanical defects are creases, lumps, that is elevation type defects which are not associated with discolorations. Such defects cannot be seen when the sheet is illuminated by omnidirectional light, or by a light source which is in line with the eye of the observer. Optical defects are for example spots, holes, or faint disclorations, that is defects which can be seen when illuminated from any angle.
On some type of materials, such as medium grade papers for printing, optical defects when small are of minor significance, whereas creases, lumps and tears are far more important for the printer because such defects can interfere with the printing and can spoil the printing blanket.
Mechanical defects can be made visible with the aid of shallow illumination which gives rise to shadows. The shallower the illumination the greater is the area of shadow produced by a given defect. Hence the optical system of the detection apparatus can detect the slightest mechanical defect if the mean angle of incidence of the illumination can be made shallow enough and the intensity of illumination high enough.
In order to obtain as high a light intensity as possible on the section of the sheet material which is observed through the aperture of a collimating optical system the lamp is placed as close as possible to the aperture and the sheet. If one now tries to reduce the angle of incidence of the illumination and at the same time maintain the distance between the aperture and the sheet one finds that this is only possible by either exposing the lamp so that it can be moved nearer the sheet, with danger of breakage of the lamp, or by moving the lamp farther away from the observed section of the sheet. As the intensity of the illumination of the sheet is inversely proportional to the square of the distance of the lamp from the inspected sheet section the latter method for achieving shallower illumination is not practical. Also it is difficult to provide sufficient space for the lamp movement as the inspection system has to be as compact as possible.